Climbing robot with compliant pinion drive

ABSTRACT

An automated order fulfillment system and mobile robot are disclosed, where the mobile robot includes a compliant drive for moving between levels of a multilevel storage structure.

CROSS-REFERENCE TO RELATED APPLICATION(S)

The present application claims priority to U.S. Provisional PatentApplication No. 62/731,300, entitled, “Climbing Robot with CompliantPinion Drive,” filed on Sep. 14, 2018, for all subject matter common toboth applications. The disclosure of said provisional application ishereby incorporated by reference in its entirety.

FIELD OF THE INVENTION

The exemplary and non-limiting embodiments described herein relategenerally to robot travel within an automated retail supply chainstorage and retrieval system, and more particularly to a compliantpinion drive on a mobile robot for tolerant meshing with a gear rackwhen the robot engages the gear rack for vertical climbing.

BACKGROUND

An automated order fulfillment system for use in supply chains mayfulfill orders for individual product items, also referred to herein as“eaches.” Traditional order fulfillment facilities store eaches incontainers in a multi-level storage structure with a vertical andhorizontal array of storage spaces. The automated order fulfillmentsystem further includes mobile robots which move around the storagestructure to transfer containers or totes to and from the storage spaceswithin the structure. In one example, the storage structure may includehorizontal floors or platforms around at least a portion of the storagestructure enabling mobile robots to travel to and from the storagestructure, horizontal tracks enabling mobile robots to travelhorizontally to and from storage spaces on a given level, and verticaltowers enabling mobile robots to travel vertically between levels.

It is known to provide pinion gears on opposed sides of the mobilerobots, which mesh with toothed racks in opposed sides of the verticaltowers to enable vertical travel. In operation, a mobile robot mayapproach a vertical tower from a horizontal floor, platform or rail.Once positioned in the vertical tower, motors in the mobile robot extendthe pinion gears on opposed sides of the mobile robot along their commonaxis of rotation until the pinion gears engage within the racks.Rotation of the pinion gears may thereafter raise or lower the robot inthe vertical tower. With this method of engagement, it may happen that apinion gear does not mesh properly with the teeth of a rack as thepinion gear advances toward and into the rack, possibly even jammingagainst a side of the rail to prevent meshed engagement.

SUMMARY

Embodiments of the present technology relate to a compliant pinion driveon a mobile robot for tolerant meshing with a rack when the robotengages the rack for vertical climbing.

In one example, the present technology relates to a mobile robotconfigured to travel in a vertical or inclined passage within anautomated retrieval and storage system, the passage comprising a rackhaving gear teeth, the mobile robot comprising: a pinion gear configuredto rotate on a shaft about an axis of rotation, the shaft configured toextend axially along the axis of rotation to position the pinion gear inmeshing engagement with the rack when the mobile robot is to travel inthe passage, the pinion gear comprising gear teeth having chamfered leadin portions configured to facilitate meshing engagement of the piniongear with the rack without jamming upon axial extension of the shaft.

In another example, the present technology relates to a mobile robotconfigured to travel in a vertical or inclined passage within anautomated retrieval and storage system, the passage comprising a lineardrive mount, the mobile robot comprising: a compliant drive assembly formoving the mobile robot in the passage, the compliant drive assemblycomprising: a motor mounted within the mobile robot, the motorcomprising a first spline; a shaft having a second spline, the shaftconfigured to be rotated about an axis of rotation by the motor by thefirst spline of the motor exerting force on the second spline of theshaft, the shaft further configured to be extended axially along theaxis of rotation; a drive gear mounted on an end of the shaft, the drivegear configured to move into engagement with the linear drive mount uponextension of the shaft; wherein the first and second splines areconfigured for rotational play between the first and second splines, therotational play allowing a degree of free rotation of the drive gearrelative to the linear drive mount to prevent jamming of the drive gearagainst the linear drive mount when the drive gear moves into engagementwith the linear drive mount upon extension of the shaft.

In a further example, the present technology relates to an orderfulfillment system, comprising: a multilevel storage structure havingstorage locations in different levels, and a vertical or inclinedpassage passing between the different levels, the passage comprising alinear drive mount along its length; and a mobile robot configured totravel between the levels in the passage, the mobile robot comprising: amotor mounted within the mobile robot, the motor comprising a firstspline; a shaft having a second spline, the shaft configured to berotated about an axis of rotation by the motor by the first spline ofthe motor exerting force on the second spline of the shaft, the shaftfurther configured to be extended axially along the axis of rotation; adrive gear mounted on an end of the shaft, the drive gear configured tomove into engagement with the linear drive mount upon extension of theshaft; wherein the first and second splines are configured forrotational play between the first and second splines, the rotationalplay allowing a degree of free rotation of the drive gear relative tothe linear drive mount to prevent jamming of the drive gear against thelinear drive mount when the drive gear moves into engagement with thelinear drive mount upon extension of the shaft.

In another example, the present technology relates to a method oftransporting a mobile robot along a vertical or inclined passage betweenstorage levels in an order fulfillment facility, the passage comprisinga linear drive mount, comprising the steps: extending a drive gear ofthe mobile robot from a first position spaced from the linear drivemount to a second position where the drive gear is engaged with thedrive mount, the drive gear mounted to a shaft; rotating the shaft anddrive gear by a motor, the motor having a first spline and the shafthaving a second spline, the shaft rotating by a force exerted by thefirst spline on the second spline; and preventing improper seating ofthe drive gear against the linear drive mount by, at least in part,providing a degree of play between the first and second splines, theplay allowing a degree of free rotation of the drive gear relative tothe linear drive mount as the drive gear extends from the first positionto the second position.

This Summary is provided to introduce a selection of concepts in asimplified form that are further described below in the DetailedDescription. This Summary is not intended to identify key features oressential features of the claimed subject matter, nor is it intended tobe used as an aid in determining the scope of the claimed subjectmatter. The claimed subject matter is not limited to implementationsthat solve any or all disadvantages noted in the Background.

BRIEF DESCRIPTION OF THE FIGURES

The present technology will now be described with reference to thefigures in which:

FIG. 1 is a side view of a climbing robot;

FIG. 2 is an isometric view of a pinion;

FIG. 3 is an isometric view of a rack structure;

FIG. 4 is a top view of a rack structure;

FIG. 5 is a side section view of a rack structure;

FIG. 6 is a partial section view of a climbing robot;

FIG. 7 is a partial section view of a climbing robot;

FIG. 8 is a partial section view of a climbing robot;

FIG. 9 is a partial section view of a climbing robot;

FIG. 10 is a partial section view of a climbing robot;

FIG. 11 is an isometric view of a shaft;

FIG. 12 is an isometric view of a hub;

FIG. 13 is an isometric view of a shaft;

FIG. 14 is an isometric view of a hub;

FIG. 15 is an isometric view of a shaft;

FIG. 16 is an isometric view of a hub;

FIG. 17 is an isometric view of a shaft;

FIG. 18 is an isometric view of a hub;

FIG. 19 is an isometric view of a hub;

FIG. 20 is an isometric view of a shaft;

FIG. 21 is an isometric view of a pinion;

FIG. 22 is an isometric view of a shaft;

FIG. 23 is a side view of an example order fulfillment system inaccordance with aspects of the disclosed embodiment;

FIG. 24 is a front view of an example order fulfillment system inaccordance with aspects of the disclosed embodiment;

FIG. 25 is a partial isometric view of an example order fulfillmentsystem in accordance with aspects of the disclosed embodiment;

FIG. 26A is a schematic illustration of a portion of storage structurein accordance with aspects of the disclosed embodiment;

FIGS. 26B-C are isometric views of a workstation in accordance withaspects of the disclosed embodiment;

FIGS. 27A, 27B, 27C, 27D, 27E, 27F and 27G represent a step-wiseprogression of the vehicle transitioning from horizontal movement tovertical movement, in accordance with aspects of the disclosedembodiment;

FIGS. 28A-28K are views of a vehicle illustrating a case or totetransfer mechanism in accordance with aspects of the disclosedembodiment;

FIG. 29 is a flow diagram in accordance with aspects of the disclosedembodiment; and

FIG. 30 is a side schematic view of a transmission in accordance withaspects of the disclosed embodiment.

DETAILED DESCRIPTION

Referring now to FIG. 1 , there is shown a side view of a climbing robot10. Climbing robot 10 is configured for use within an order fulfillmentsystem and may have features as described in US Patent PublicationNumber US 2017/0313514 A1 dated Nov. 2, 2017 and entitled “OrderFulfillment System” which is incorporated by reference herein in itsentirety. An order fulfillment system has multilevel tote storage andretrieval structure, autonomous robotic vehicles 10 configured to pick,transport and place one or more tote within the order fulfillmentsystem, workstations configured to accommodate a picker (human,automated or otherwise) that transports one or more eaches from a tote,for example a product tote containing multiple common eaches to bepicked, on one of the autonomous mobile robots to a put location, forexample an order tote that has a combination of different eaches thatreflects a full or partially fulfilled order, that may be on another ofthe autonomous mobile robots at the workstation, transit decksconfigured to support, stage and buffer the autonomous robots betweenthe storage and retrieval structure and the workstations, a dispensestation where totes containing fulfilled orders are discharged from theorder fulfillment apparatus and a decant or input interface (not shown)configured to replenish the apparatus. Climbing robot 10 has drivewheels 12, support wheels 14, guide rollers 16, pinion 18 andcounterwheel 19 and counterwheel 20. As will be described in greaterdetail below, pinion 18 and counterwheel 19 and counterwheel 20selectively engage a rack allowing the robot 10 to climb or descendvertically. While climbing, counterwheel 19 and counterwheel 20 react tothe cantilevered weight of robot 10 having CG 22 where force 24 reactswith counterwheel 20 and force 26 reacts with counterwheel 19. Pinion 18further vertically supports the weight 28 of robot 10 while thecounterwheels react the moment.

Referring now to FIG. 2 , there is shown an isometric view of pinion 18.Pinion 18, also referred to herein as a drive gear, has teeth 40 andkeyway 42. Teeth 40 have a chamfered lead in portion 44 that is providedto prevent jamming or missed engagement when engaging the rack.

Referring now to FIG. 3 , there is shown an isometric view of rackstructure 80. Referring also to FIG. 4 , there is shown a top view ofrack structure 80. Referring also to FIG. 5 , there is shown a partial aside section view of rack structure 80. Rack structure 80 has rack 82(also referred to herein as a linear drive mount), counter wheel supportsurfaces 84, 86, robot support rails 88, 90 and frame 92. The teeth ofrack 82 have a chamfered lead in portion 94 that is provided to preventjamming or missed engagement when the pinion engages rack 82. Cut outs96, 98 are positioned where the counterwheels engage the rack structure.Backer bar surface 102 is provided such that an engaged pinion cannotjump or skip rack teeth due to the pinion hitting backer bar 102 priorto disengaging the rack teeth. Counter wheel support surfaces 84, 86 areprovided as a vee-wheel track used to center vee-wheels as will bedescribed in greater detail, where by example, surface 86 has a flatbottom portion 98, for example to engage a mating surface on acounterwheel to set the pinion gear teeth engagement without bottomingout the pinion gear, serves as lead in of pinion gear into therack/backer bar space. Vee portion 100 is further provided to mate withmating chamfered portions of a counterwheel to center the wheel ingroove 86.

Referring now to FIG. 6 , there is shown a partial section view of aclimbing robot 10. Racks 82, 82′ are positioned on opposing sides ofrobot 10. Similarly, support rails 90, 90′ are positioned on opposingsides of robot 10. In FIG. 6 , the robot is shown with drive wheels 12,12′ supported by support rails 90, 90′. As will be described withrespect to FIGS. 6-10 , robot 10 will engage racks 82, 82′ to climbvertically. Wheels 12, 12′ are independently driven by motors 120, 120′.Independently driven motors 120, 120′ are shown grounded to the frame ofrobot 10 where independently driven motors 120, 120′ transmit torque toshafts 122, 122′ via spline or bushing 124, 124′ where pinions 18, 18′and counterwheels 19, 19′ are coupled to shafts 122, 122′ Similarlyshafts 122, 122′ transmit torque to wheels 12, 12′ via spline or bushing126, 126′. Counter wheels 20, 20′ are coupled to shafts 130, 130′respectively where trunnion 132 couples shafts 122 and 130 and wheretrunnion 132′ couples shafts 122′ and 130′. Motor 136 is providedcoupled to opposite hand lead screws 138, 138′ such that rotation ofmotor 136 causes shafts 122, 130 to move in opposition to shafts 122′,130′ to selectively engage and disengage the racks 82, 82′. It is notedthat counterwheels 19, 20 have mating surfaces that engage counter wheelsupport surfaces 84, 86 that are provided as a vee-wheel track is usedto center vee-wheels or counterwheels 19, 20. After engagement, servomotor or motor 136 can be disabled and the lead screws can be allowed toback drive such that the counterwheels 19, 20 mating surfaces engage andfollow counter wheel support surfaces 84, 86 and by monitoring anencoder on motor 136 the encoder position can be recorded duringvertical or other motion to measure the separation of the counter wheeltracks (eg. 86, 86′) in the storage system. In addition to a monitoringand measurement function, limits may be set, for example if theseparation of the counterwheels is less than a predetermined minimumthen the motor 136 may be reengaged pushing out the counterwheels. Asimilar feature may be provided for wheels 12, 12′, for example, tomeasure the width of the track or set limits or otherwise. Here andsimilarly, a motor may be provided coupled to opposite hand lead screws(not shown) such that rotation of the motor causes trucks 140, 140′ andhence wheels 12, 12′ to move in opposition to selectively engage anddisengage the support rails 90, 90′. As will be described in greaterdetail, shafts 122, 122′ are provided with features such that there isprovided play between shaft 122 and wheel 12 via spline 126 such thatupon the pinion 18 engagement with rack 82 the system is not overconstrained. By way of example, were play not provided if the pinion wasout of alignment with the rack, the friction between the wheel and thewheel support would prevent smooth engagement. In the relative positionbetween spline 126 and shaft 122 shown in FIG. 6 , there is norotational play between spline 126 and shaft 122.

Referring now to FIG. 7 , there is shown a partial section view of aclimbing robot 10 showing one side for clarity. In the state shown inFIG. 7 , lead screw 138 is rotated such that pinion 18 is just about toengage with rack 82. In the relative position between spline 126 andshaft 122 shown in FIG. 7 , there is rotational play between spline 126and shaft 122 such that pinion 18 can freely engage rack 82.

Referring now to FIG. 8 , there is shown a partial section view of aclimbing robot 10 showing one side for clarity. In the state shown inFIG. 8 , lead screw 138 is rotated such that pinion 18 is fully engagedwith rack 82. In the relative position between spline 126 and shaft 122shown in FIG. 8 , there is rotational play between spline 126 and shaft122 such that pinion 18 can freely engage rack 82. In the event pinion18 fails to engage rack 82, a retry feature may be provided. Here, inthe event there is an interference/overlap of the pinion and rack gearteeth edges, the pinion may be retracted and rotated using the drivemotor (while still in the compliant portion of the shaft), and thepinion reinserted back into the rack. By way of example, the pinion maybe rotated slightly forward (driving up) and then reinserted for asuccessful engagement of pinion 18 with rack 82. Note that the sequencereferred to with respect to FIGS. 6-10 may be reversed to disengage thepinion and engage the drive wheels for subsequent horizontal movementwith the drive wheels.

Referring now to FIG. 9 , there is shown a partial section view of aclimbing robot 10 showing one side for clarity. In the state shown inFIG. 9 , pinion 18 is fully engaged with rack 82 and motor 120 appliestorque to shaft 122 and pinion 18 such that the climbing robot is raisedseparating wheel 12 from support rail 90. In the relative positionbetween spline 126 and shaft 122 shown in FIG. 9 , there is rotationalplay between spline 126 and shaft 122 such that wheel 12 can freelyseparate from rail 90.

Referring now to FIG. 10 , there is shown a partial section view of aclimbing robot 10 showing one side for clarity. In the state shown inFIG. 10 , pinion 18 is fully engaged with rack 82 and motor 120 appliestorque to shaft 122 and pinion 18 such that the climbing robot is fullysupported by pinion 18 and counter wheels 19, 20. Truck 140 is retractedby a separate motor (not shown) such that wheel 12 clears support rail90. In the relative position between spline 126 and shaft 122 shown inFIG. 10 , there is no rotational play between spline 126 and shaft 122such that as the wheel is stopped there is no play such that duringdeceleration the wheel remains centered on the compliance zone.

Referring now to FIG. 11 , there is shown an isometric view of a shaft122. Referring also to FIG. 12 , there is shown an isometric view of aspline hub 126. Spline hub 126 mounts to wheel 12 and moves axially onshaft 122. Shaft 122 has pinion mounting portion 160 with keyway 162 fortransmitting torque to the pinion 18. Shaft 122 further has counterwheelmounting portion 164 and splined portion 166. Splined portion 166 hasportions 168, 172 that mate with internal spline 174 of hub 126 suchthat when hub 126 mates with portions 168, 172 there is little to norotational play between hub 126 and shaft 122. Splined portion 166further has portion 170 that mates with internal spline 174 of hub 126such that when hub 126 mates with portion 170 there is a predeterminedrotational play between hub 126 and shaft 122 where the external splinehas been narrowed 176. Lead ins 178, 180 are provided to allow a smoothtransition between the hub 126 and portions 168, 170, 172.

Referring now to FIG. 13 , there is shown an isometric view of analternate embodiment shaft 122″. Referring also to FIG. 14 , there isshown an isometric view of an alternate embodiment ball spline hub 126″.Ball spline hub 126″ mounts to wheel 12 and moves axially on shaft 122″.Ball spline hub 126″ has recirculating balls 200, 202. Shaft 122″ haspinion mounting portion 160 with keyway 162 for transmitting torque tothe pinion 18. Shaft 122 further has counterwheel mounting portion 164and grooved portion 204. Grooved portion 204 has race portions 206, 210that mate with the recirculating balls 200, 202 of hub 126″ such thatwhen hub 126″ mates with portions 200, 202 there is little to norotational play between hub 126″ and shaft 122″. Grooved portion 204further has portion 208 that mates with the recirculating balls 200, 202of hub 126″ such that when hub 126″ mates with portion 208 there is apredetermined rotational play between hub 126″ and shaft 122″ where theexternal race has been widened 212. Lead ins 214, 216 are provided toallow a smooth transition between the hub 126″ and portions 206, 208,210.

Referring now to FIG. 15 , there is shown an isometric view of analternate embodiment shaft 122′″. Referring also to FIG. 16 , there isshown an isometric view of an alternate embodiment square spline or hub126′″. Square hub 126′″ mounts to wheel 12 and moves axially on shaft122′″. Square hub 126′″ a mating square portion 240 with lead in chamfer242. Shaft 122′″ has pinion mounting portion 160 with keyway 162 fortransmitting torque to the pinion 18. Shaft 122 further has counterwheelmounting portion 164 and spline or square portion 246. Square portion246 has portions 248, 252 that mate with the mating square portion 240of hub 126′″ such that when hub 126′″ mates with portions 248, 252 thereis little to no rotational play between hub 126′″ and shaft 122′″.Square portion 246 further has portion 250 that mates with mating squareportion 240 of hub 126′″ such that when hub 126′″ mates with portion 250there is a predetermined rotational play between hub 126′″ and shaft122′″ where the external square portion 250 has been narrowed 256. Leadins 258, 260 are provided to allow a smooth transition between the hub126′″ and portions 248, 250, 252.

Referring now to FIG. 17 , there is shown an isometric view of analternate embodiment shaft 300. Referring now to FIG. 18 , there isshown an isometric view of an alternate embodiment hub 320. Shaft 300 isshown to mate with a hub 320 having a keyway 322. Shaft 300 is shownwith pinion 18 and counterwheel 19. It is noted that the motor drivespline or bushing and wheel drive spline or bushing ride on the shaftand may have similar features as hub 320 (or hubs as described). Shaft300 further has a key 302 with key portions 304, 308 that mate with themating keyway of the keyed hub such that when the keyed hub mates withportions 304, 308 there is little to no rotational play between the huband shaft 300. The key is modified to have a necked down region that isused to provide compliance where key 302 further has portion 306 thatmates with the keyway of the hub 320 such that when the hub 320 mateswith portion 306 there is a predetermined rotational play between thehub and shaft 300 where the external key 302 has been narrowed 310 atportion 306. Lead ins 312, 314 are provided to allow a smooth transitionbetween the hub and portions 304, 306, 308. Hub 320 has linear bearing324, housing 326 and insert 328 having key 322. Linear bearing orbushing 324 may be a sleeved or recirculating ball bushing and providesthe extend and retract action of the bearing on the shaft. Housing 326holds the linear bearing 324 and also holds the insert 328 that may bebronze, a polymer such as nylon or otherwise and having key way 322.Insert 328 transmits torque to the shaft key 302 and has secondarykeyway 330 that transmits torque from the motor to the shaft or theshaft to the hub. Here, when the bronze insert in the motor or hubbushing travels over the necked down region of the key, compliance isintroduced. This embodiment has similar behavior to the embodiment shownin FIG. 13 or otherwise but does not have rolling balls running over thecompliant region.

Referring now to FIG. 20 , there is shown an isometric view of shaft340. Shaft 340 has race grooves 342 configured to mate with arecirculating ball spline hub. Alternately, any suitable hub or splinemay be used to transmit torque. Shaft 340 further has pinion 344 andcounterwheel 346. Shaft 340 further has opposing keys 348, 350 that matewith keyways 352, 354 of pinion 344 respectively. Keyways 352, 354 arecut with additional clearance 356 to introduce a predetermined amount ofplay between the shaft and the pinion. Features are provided to keep thekey centered in the keyway when torque is not being transmitted. Pinion344 has a recessed rectangular region 360 that accepts a torsionalspring 362. Spring 362 has legs 364 on opposing sides of the shaft thatengage the walls of recess 360 in pinion 344. Torsional spring 362further has a tongue 366 that engages a slot 368 in the shaft. Torque istransmitted through the shaft to the tongue via the slot and to thepinion via the compliant legs 364 of the torsional spring 362 such thatcompliance is introduced between the shaft and the pinion until the key348 engages keyway 352.

FIGS. 23-30 , and the accompanying description, provide an overview ofan automated retrieval and storage system in which the compliant pinionrobot 10 may be used. FIGS. 23, 24, 25, 26A, 26B-C, 27A-G, 28A-K, 29 and30 correspond directly to FIGS. 15, 16A, 17, 54G, 54A-B, 60A-G, 52A-K,55 and 40A of US Patent Publication Number US 2017/0313514 A1 dated Nov.2, 2017 and entitled “Order Fulfillment System,” which publication hasbeen incorporated by reference herein and in the parent provisionalpatent application in its entirety. The following description of FIGS.23, 24, 25, 26A, 26B-C, 27A-G, 28A-K, 29 and 30 correspond directly toat least portions of the description of FIGS. 15, 16A, 17, 54G, 54A-B,60A-G, 52A-K, 55 and 40A of US Patent Publication Number US 2017/0313514A1.

Referring now to FIGS. 23-25 , there are shown side, front and top viewsrespectively of example order fulfillment system configured in a vendingconfiguration. Here, the order fulfillment system may be described as anorder vending machine 420 or “OVM” or otherwise. The order vendingmachine 420 shows an alternate, for example, scaled down version of therobotic vehicle and rack system, for example, that may be utilized instore vending of delivered goods or any other suitable application. Forexample, the vehicle technology may be used in e-commerce as applied tothe “last-mile” delivery problem. For example, “Pure-play” e-commercecompanies have little choice but to deliver the vast majority of ordersto customers' homes, which may be costly. Retailers who both operateself-service stores and sell online can offer customers the choice ofpicking orders up at store locations, commonly called“click-and-collect”, but in practice this model places an additional andunpredictable workload on store personnel that may result in extendedwait times by customers, etc. Here, the order vending machine 420provides an automated solution that requires a very little floor space(or land) but can securely hold a large number of orders, and alsoprovides convenient on-demand access and short transaction times tocustomers. Here, the order vending machine 420 may be a robotic vehiclebased “micro-warehouse” that may be referred to as an Order VendingMachine (OVM) that operates in conjunction with an e-commercefulfillment center, for example one equipped with a robotic vehiclebased system. In one aspect, Order-Totes (“O-Totes”) containing customerorders may be delivered to and stored within the OVM, and then presentedon demand to customers, with robotic vehicles performing all requiredTote-storage and retrieval functions.

Referring now to FIGS. 26A, 26B and 26C, an order fulfillmentworkstation 5500 is shown. While one workstation 5500 is shown in FIGS.26B and 26C, it should be understood that the storage and structure 5563(which is substantially similar to the storage structures describedherein) may have any suitable number of workstations 5500. For example,FIG. 26A illustrates an exemplary configuration of workstations 5500where at least three workstations 5500 are disposed on each storagelevel, while in other aspects any suitable number of workstations may bedisposed on each storage level. The workstations 5500 for the differentlevels may be vertically offset from one another such as being stackedone above the other or stacked in a staggered arrangement. In oneaspect, each workstation 5500 is communicably connected to two transitdecks 5550A, 5550B, while in other aspects each workstation 5500 may becommunicably connected to any suitable number of transit decks. In oneaspect, each transit deck 5550A, 5550B may correspond to a respectivestorage level while in other aspects the transit decks 5550A, 5550B maycorrespond to a common storage level (e.g. there is more than onetransit deck associated with each storage/picking level). In anotheraspect, there may be towers (substantially similar to elevation tracks5190) that are located on or otherwise connected to (or disposed within)the transit decks (or aisles) that communicably connect one or more ofthe transit decks 5550A, 5550B (or aisles) of the different storagelevels to from a travel loop with another tower so that bots 5100 maytraverse between the stacked transit decks 5550A, 5550B (or aisles) toany desired/predetermined level of the storage structure. Theworkstations 5500 are configured to accommodate a picker 5520 thattransports one or more eaches from a tote (e.g. a P-tote) on one of thebots 5100 to a “put” location in a tote (e.g. an O-tote) on another oneof the bots 5100. The workstations 5500 may be arrayed at multipleelevations where human or robotic pickers remove eaches from productTotes (P-totes) and place them into either order Totes (O-totes) or amobile robot, depending on the system configuration and in a mannersubstantially similar to that described above. In one aspect, theworkstation 5500 includes conveyance lanes or aisles 5501, 5502, 5503,5504, elevation towers 5190T and a picker platform 5510 disposed at apick station 5530. A workstation 5500 is disposed at each transit decklevel so that bots 5100 on each transit deck have access to aworkstation 5500. In the exemplary aspect illustrated in FIG. 26B twotransit deck levels 5550A, 5550B are shown connected to a commonworkstation 5500 however, in other aspects any suitable number oftransit deck levels may be connected to a common workstation 5500.

Each of the conveyance lanes 5501, 5502, 5503, 5504 has a respectiveentry and/or exit 5500E in communication with a respective transit deck5550A, 5550B. As can be seen in FIG. 26B conveyance lanes 5501, 5504have entry/exits 5500E in communication with transit deck 5550B whileconveyance lanes 5502, 5503 have entry/exits 5500E in communication withtransit deck 5550A. The conveyance lanes 5501-5504 include rails WRRthat are substantially similar to rails HRR described above with respectto the aisles providing access to the tote storage/holding locations. Ascan also be seen in FIG. 26B elevation towers 5190TWA-5190TWD connectstacks of conveyance lanes to each other in a manner substantiallysimilar to that described above with respect to elevation towers 5190T.The elevation towers 5190TWA-5190TWD are substantially similar to theelevation towers 5190T described above. As an example, elevation towers5190TWA, 5190TWB connect conveyance lanes 5503, 5504 so that bots 5100can traverse between the conveyance lanes 5503, 5504. Elevation towers5190TWC, 5190TWD connect conveyance lanes 5501, 5502 so that bots 5100can traverse between the conveyance lanes 5501, 5502.

In one aspect, one or more of the conveyance lanes 5501-5504 and towers5190TWA-5190TWD may be angled (e.g. tilted or raked) relative to thetransit decks 5550A, 5550B and the operator platform 5510 so that whenthe P-totes and O-totes are presented to the picker 5520 by therespective P-bot and O-bot, the P-totes and O-totes are angled so thatthe picker 5520 can view and access the P-totes and O-totes for pickingand placing eaches from pick/place positions defined by the towers5190TWA, 5190TWC adjacent the pick station 5530. In other aspects, theconveyance lanes 5501-5504 and towers 5190TWA-5190TWD may have anyspatial relationship with the pick station 5530 and/or transit decks5550A, 5550B for presenting the totes to the picker 5520 in any suitablespatial orientation.

In one aspect, the conveyance lanes 5501-5504, the elevation towers5190TWA-5190TWD and the pick station 5530 have a symmetric structurewith independent product bots (P-bots) and order bots (O-bots) paths andpositions. In this aspect there may be lateral symmetry (in direction5599) so that there is a left/right symmetrical arrangement. Forexample, the left/right symmetrical arrangement may be such that P-botscarrying P-totes are arranged on the right side of the workstation 5500while O-bots carrying O-totes are arranged on the left side of theworkstation 5500. In other aspects, the P-bots and P-totes may be on theleft side of the workstation 5500 while the O-bots and O-totes are onthe right side of the workstation 5500.

In one aspect, there are dedicated bot flow entry and exit conveyancelanes for both the P-bots and O-bots. For example, the flow of bots tothe pick station 5530 may be such that the bots travel from lowerconveyance lanes to upper conveyance or in other aspects, from upperconveyance lanes to lower conveyance lanes. For example, where botstravel from lower conveyance lanes to upper conveyance lanes, P-botscarrying eaches to be picked enter one or more lower/bottom conveyancelane(s) 5501, traverse tower 5190TWC to one or more upper conveyancelane(s) 5502 so that the each(es) can be picked where the P-bot exitsthe workstation using the one or more upper conveyance lane(s) 5502.Similarly, e.g., O-bots carrying O-totes to which eaches are to beplaced enter one or more lower/bottom conveyance lane 5504, traversetower 5190TWA to one or more upper conveyance lane(s) 5503 so that theeach(es) can be placed where the O-bot exits the workstation using theone or more upper conveyance lane(s) 5503. In other aspects, theentrance of bots to the workstation may be timed such that the bots canenter and exit from both the upper conveyance lanes 5502, 5503 and thelower conveyance lanes 5501, 5504 where the towers 5190TWA-5190TWD areemployed to route bots past one another such as when bots are enteringand exiting a common conveyance lane 5501-5504. In the examples,described above, the flow of P-bots carrying P-totes and the flow ofO-bots carrying O-totes are both generally in a common direction, suchas both in the direction of arrow 5598 from lower conveyance lanes toupper conveyance lanes or both in the direction of arrow 5597 from upperconveyance lanes to lower conveyance lanes. However, in other aspects,the flow of one or more of the P-bots and O-bots may be in the directionof arrow 5597 from upper conveyance lanes to lower conveyance lanes. Forexample, the flow of P-bots and P-totes may be in the direction 5598while the flow of O-bots and O-totes may be in the direction 5597 orvice versa.

In one aspect, each side of the workstation 5500 (e.g. the product sideand the order side) has dedicated flow direction elevation towers. Forexample, elevation tower 5190TWC on the product side of the workstation5500 may be dedicated to the upward flow of bots while elevation tower5190TWD on the product side of the workstation 5500 may be dedicated tothe downward flow of bots or vice versa. Similarly, elevation tower5190TWA on the order side of the workstation 5500 may be dedicated tothe upward flow of bots while elevation tower 5190TWB on the order sideof the workstation 5500 may be dedicated to the downward flow of bots orvice versa. The dedicated flow of bots for each tower 5190TWA-5190TWD onthe respective order or product side of the workstation 5500 generates,for example, an elevation flow loop in one or more of directions 5597,5598 between the levels of conveyance lanes 5501-5504 on the respectiveorder and product sides of the workstation 5500 in a mannersubstantially similar to that described above. As noted above, whileonly two conveyance lanes are shown stacked one above the other on eachside of the workstation, in other aspects each side of the workstationmay have any suitable number of conveyance lanes stacked one above theother, such as more or less than two conveyance lanes. Where more thantwo conveyance lanes are provided, stacked one above the other, on theproduct side and/or the order side of the workstation 5500 the towers5190TWA-5190TWD may have intermediate entrance and exits that allow botsto enter/exit the towers from the intermediate conveyance lanes ILdisposed between the uppermost and lowermost conveyance lanes 5502, 5501of the stack of conveyance lanes.

As described above, the towers 5190TWA, 5190TWC adjacent the pickstation 5530 define the pick/place positions of the bots (e.g. theP-totes and O-totes). For example, the pick positions may be defined bythe towers 5190TWA, 5190TWC so as to be substantially at the top of thetowers 5190TWA, 5190TWC at a position along the tower that allows thebots to transition between the towers 5190TWA, 5190TWC and therespective uppermost conveyance lane 5502, 5503. As also describedabove, the towers 5190TWA, 5190TWC may be angled relative to the pickstation 5530 for presenting the P-totes and O-totes to the picker 5520in any suitable spatial orientation. In one aspect, the other towers5190TWB, 5190TWD (e.g. disposed along the conveyance lanes 5501-5504 onan opposite side of the towers 5190TWA, 5190TWC, that define thepick/place positions, from the picker 5520) that form the elevation loopwith a respective one of the towers 5190TWA, 5190TWC may be angled atthe same angle as the towers 5190TWA, 5190TWC or angled at any suitabledifferent angle relative to the pick station 5530. In one aspect, thetowers 5190TWB, 5190TWD may be substantially upright (e.g. vertical). Inone aspect, as described above, the conveyance lanes 5501-5504 may alsobe angled with respect to the pick station so as to form ramps betweenthe transit decks 5550A, 5550B (and/or intermediate decks IL) and theoperator platform 5510 where the ramps allow for substantiallyorthogonal alignment between one or more of the towers 5190TWA-5190TWDand the respective conveyance lanes 5501-5504 to facilitate ease ofconstraints and repeat engagement for bot 5100 transitions from thetowers 5190TWA-5190TWD to the conveyance lanes 5501-5504 and vice versa.In one aspect the tower rake/angle establishes or defines the ramppitch.

In one aspect, the workstation 5500 includes any suitable Machine-VisionSubsystem (“MVS”) 5560 which may be substantially similar to thatdescribed above. For example, the MVS 5560 may include any suitablevisual indicators (e.g. such as displays and/or light sources), anysuitable aural indicators, any suitable motion sensors/cameras, anysuitable beam sensors/light curtains (e.g. break the beam/curtainsensors), glove tracking systems or any other suitable devices forindicating a pick location, indicating a place location, indicating aquantity to be picked/placed, tracking the motion of the picker's 5520hands, verifying a pick and/or verifying a place. In one aspect, acontroller 5500C is provided for controlling the aspects of theworkstation 5500 described herein where the controller 5500C is residentat the workstation 5500, a central control system CCS (such as describedabove), a bot 5100 controller or a combination thereof. The controller5500C communicates with the machine vision system 5560 to effect thepicking and placing of eaches as described herein.

In one aspect, the controller 5500C is configured to identify andvalidate an effective pick where the controller 5500C issues aconfirmation to a P-bot that a pick has been effected from the P-bot.The P-bot is configured (e.g. the P-bot controller is programmed) suchthat when the P-bot receives the pick confirmation issued from thecontroller 5500C, the P-bot automatically moves from the pick station5530. In one aspect, the P-bot controller may be programmed such thatupon receiving the pick confirmation the P-bot traverses to an exit of arespective conveyance lane 5502 or in other aspects, the P-bot entersinto a tower, such as tower 5190TWD, to return back to the entryconveyance lane 5501 for re-entry into the pick queue.

In one aspect, the controller 5500C is configured to identify andvalidate an effective place where the controller 5500C issues aconfirmation to an O-bot that a place has been effected to the O-bot.The O-bot is configured (e.g. the O-bot controller is programmed) suchthat when the O-bot receives the place confirmation issued from thecontroller 5500C, the O-bot automatically moves from the pick station5530. In one aspect, the O-bot controller may be programmed such thatupon receiving the place confirmation the O-bot traverses to an exit ofa respective conveyance lane 5503 or in other aspects, the O-bot entersinto a tower, such as tower 5190TWB, to return back to the entryconveyance lane 5504 for re-entry into the place queue. In one aspect,the O-bot may re-enter into the place queue until the controller 5500Cissued an order complete command to the O-bot at which time the O-botexits the pick station 5530 to the transit deck(s).

FIGS. 27A, 27B, 27C, 27D, 27E, 27F, and 27G represent a step-wiseprogression of the vehicle transitioning from horizontal movement tovertical movement, in accordance with aspects of the disclosedembodiment. Specifically, the vehicle in the form of the bot 5100 isshown, which as indicated elsewhere herein may be substantially similarto the bots described throughout this disclosure unless otherwise noted.

In FIGS. 27A through 27C, the bot 5100 travels horizontally from rightto left across the page. At FIGS. 27D to 27E, the drive gear 5140B (andnot shown, but on the opposite side drive gear 5140A) and guide bearingsare extended laterally away from the body of the bot 5100 to engage withthe elevation track 5190. Each elevation track 5190 has a drive surface5190D and a guide surface 5190B disposed opposite the drive surface5190D and separated by a thickness of the elevation track 5190, and thedrive surface includes a gear rack 5195, as described and shownelsewhere herein.

In FIGS. 27F to 27G, the bot 5100 travels vertically up the elevationtrack 5190.

Referring to FIGS. 28A-28J an exemplary operation of a tote 5300transfer between a tote holding location 5350 and the bot 5100 will beprovided. In one aspect the bot 5100 traverses horizontal rails HRR to apredetermined tote holding location 5350 (FIG. 29 , Block 5800). The bot5100 may include any suitable sensors or odometry to facilitate locationdetermination of the bot 5100 relative to the tote holding location5350. When the tote 5300 is substantially aligned with the payload area5180 the flippers 5230B, 5240B (and flippers 5230A, 5240A) are rotatedin direction 5399 so that the flippers extend underneath and behind therespective catch surface 5300C1, 5300C2 to cam the tote 5300 into thepayload area 5180 of the bot 5100 as described above with respect toFIG. 28K (FIG. 29 , Block 5802). The flippers 5230B, 5240B continue tomove in direction 5399 to pull the tote laterally in direction 5999Afrom the tote holding location 5350 to the payload area 5180 of the bot5100 until the tote 5300 is located at a predetermined loaded positionwithin the payload area 5180 as shown in FIG. 28D (FIG. 29 , Block5804). In one aspect, the bot 5100 includes any suitable sensors forsensing a position of the tote 5300 relative to the payload area 5180for stopping movement of the recirculating bidirectional traversers5230T, 5240T when the tote 5300 reaches the predetermined positionwithin the payload area 5180. To transfer the tote 5300 from the bot tothe tote holding location 5350 from the same side of the bot 5100 thatthe tote was transferred onto the bot 5100, the flippers 5230B, 5240B(and flippers 5230A, 5240A) are moved in direction 5398 so that flippers5230B, 5240B contact respective catch surface 5300C3, 5300C4 of the tote5300 (FIG. 29 , Block 5806). The flippers 5230B, 5240B continue to movein direction 5398 so that the flippers 5230B, 5240B push the tote 5300laterally in direction 5999B until the flippers 5230B, 5240B disengagethe respective catch surface 5300C3, 5300C4 of the tote 5300 so that thetote 5300 is partially pushed off of the bot 5100 (FIG. 29 , Block5808). The flippers 5230A, 5240A (and flippers 5230B, 5240B) continue tomove in direction 5398 so that flippers 5230A, 5240A engage respectivecatch surface 5300C1, 5300C2 where further movement of the flippers5230A, 5240A in direction 5398 pushes the tote 5300 laterally indirection 5999B off of bot 5100 where the tote 5300 is finallypositioned in the tote holding location 5350 when the flippers 5230A,5240A disengage the respective catch surfaces 5300C1, 5300C2 as theflippers 5230A, 5240A move in direction 5398 (FIG. 29 , Block 5810). Theflippers 5230A, 5230B, 5240A, 5240B are moved in direction 5399 ordirection 5398 to the starting or home position of the flippers 5230A,5230B, 5240A, 5240B as shown in FIG. 28J, where the home position may bealong the centerline CL of the bot 5100 or at any other suitableposition along the path of travel of the flippers 5230A, 5230B, 5240A,5240B (FIG. 29 , Block 5812).

Where the tote 5300 is to be pushed off of the opposite lateral side thebot 5100 than the lateral side from which the tote 5300 was pushed ontothe bot 5100, referring to FIG. 28D, the flippers 5230B, 5240B (whichare engaged with respective catch surfaces 5300C1, 5300C2) move indirection 5399 to partially push the tote 5300 off of the bot indirection 5999A until the flippers 5230B, 5240B disengage the respectivecatch surfaces 5300C1, 5300C2 (FIG. 29 , Block 5814). The flippers5230A, 5240A (along with flippers 5230B, 5240B) continue to move indirection 5399 so that the flippers 5230A, 5240A engage the respectivecatch surface 5300C1, 5300C2 to push the tote 5300 in direction 5999Auntil the flippers 5230A, 5240A disengage the respective catch surfaces5300C1, 5300C2 to finally position the tote 5300 at a tote holdinglocation (FIG. 29 , Block 5816) in a manner substantially similar tothat described above and the flippers 5230A, 5240A, 5230B, 5240B arereturned to the starting position (FIG. 29 , Block 5812).

Referring also to FIG. 30 , there is shown a side schematic view of awheel with a drive gear such as sprocket 1080 engaging a ramp. Here,counter bearing 1082 engages counter rail 1084 while chain 1086 (alsoreferred to herein as a linear drive mount) is engaged by sprocket 1088.FIG. 30 shows initial engagement where a rubber backing may be providedto enable chain meshing and limit engagement wear. It is understood thatsprocket 1088 may be mounted on shaft 122 as shown in FIG. 8 . Asdescribed above, with the relative position between spline 126 and shaft122 shown in FIG. 8 , there is rotational play between spline 126 andshaft 122 such that sprocket 1088 can freely engage chain 1086.

We claim:
 1. A mobile robot configured to travel in a vertical orinclined passage within an automated retrieval and storage system, thepassage comprising a linear drive mount, the mobile robot comprising: acompliant drive assembly for moving the mobile robot in the passage, thecompliant drive assembly having: a hub for supporting a wheel, the hubcomprising a first spline; a shaft having a second spline, the shaftconfigured to be rotated about an axis of rotation by a motor, the shaftfurther configured to be extended axially along the axis of rotation; adrive gear mounted on an end of the shaft, the drive gear configured tomove into engagement with the linear drive mount upon extension of theshaft; wherein the first and second splines are configured to preventjamming of the drive gear against the linear drive mount when the drivegear moves into engagement with the linear drive mount upon extension ofthe shaft, the first and second splines configured to prevent thejamming by providing rotational play between the first and secondsplines, the rotational play allowing a degree of free rotation of thedrive gear relative to the linear drive mount.
 2. The mobile robot ofclaim 1, wherein the mobile robot is configured to move up or downwithin the passage by the rotation of the drive gear against the lineardrive mount.
 3. The mobile robot of claim 1, wherein the drive gearcomprises a pinion gear and the linear drive mount comprises a rackcomprising teeth configured to mesh with the gears of the pinion.
 4. Themobile robot of claim 3, wherein the pinion gear comprises gear teethhaving chamfered lead in portions configured to facilitate meshingengagement of the pinion gear with the rack without jamming upon axialextension of the shaft.
 5. The mobile robot of claim 1, wherein thedrive gear comprises a sprocket and the linear drive mount comprises achain, the teeth of the sprocket configured to mesh with the links ofthe chain.
 6. The mobile robot of claim 1, wherein the linear drivemount is mounted along a track in the passage, the mobile robot furthercomprising a counter wheel mounted on an end of the shaft adjacent thedrive gear, the counter wheel configured to ride within a groove formedin the track to keep the mobile robot centered in a direction transverseto its direction of travel.
 7. The mobile robot of claim 1, whereinlinear drive mount comprises a first linear drive mount on a first sideof the passage, the passage further comprising a second linear drivemount on a second side of the passage opposite the first side, andwherein the compliant drive assembly comprises a first compliant driveassembly, the motor comprises a first motor, the shaft comprises a firstshaft and the drive gear comprises a first drive gear, the mobile robotfurther comprising: a second compliant drive assembly for moving themobile robot in the passage, the compliant drive assembly comprising: asecond motor mounted within the mobile robot, the second motorcomprising a third spline; a second shaft having a fourth spline, thesecond shaft configured to be rotated about an axis of rotation by thesecond motor by the third spline of the second motor exerting force onthe fourth spline of the second shaft, the second shaft furtherconfigured to be extended axially along the axis of rotation; a seconddrive gear mounted on an end of the second shaft, the second drive gearconfigured to move into engagement with the second linear drive mountupon extension of the second shaft; wherein the third and fourth splinesare configured for rotational play between the third and fourth splines,the rotational play allowing a degree of free rotation of the seconddrive gear relative to the second linear drive mount to prevent jammingof the second drive gear against the second linear drive mount when thesecond drive gear moves into engagement with the second linear drivemount upon extension of the second shaft.
 8. The mobile robot of claim7, wherein the first and second shafts share the same axis of rotation.9. The mobile robot of claim 8, wherein the first and second lineardrive mounts are mounted along first and second tracks in opposed sidesof the passage, the mobile robot further comprising first and secondcounter wheels mounted on opposed ends of the first and second shafts,the first and second counter wheels configured to ride within first andsecond grooves formed in the first and second tracks to keep the mobilerobot centered in a direction transverse to its direction of travel. 10.The mobile robot of claim 9, further comprising: a third motor; andfirst and second opposite hand lead screws driven by the third motor,rotation of the opposite hand lead screws extending the first and secondshafts axially in opposite directions to position the first and seconddrive gears in the first and second linear drive mounts and to positionthe first and second counter wheels in the first and second grooves. 11.The mobile robot of claim 10, further comprising an encoder on the thirdmotor to measure how far apart the first and second counter wheels arewhile riding in the first and second grooves.
 12. An order fulfilmentsystem, comprising: a multilevel storage structure having storagelocations in different levels, and a vertical or inclined passagepassing between the different levels, the passage comprising a lineardrive mount along its length; and a mobile robot configured to travelbetween the levels in the passage, the mobile robot having: a motormounted within the mobile robot, the motor comprising a first spline; ashaft having a second spline, the shaft configured to be rotated aboutan axis of rotation by the motor by the first spline of the motorexerting force on the second spline of the shaft, the shaft furtherconfigured to be extended axially along the axis of rotation; a drivegear mounted on an end of the shaft, the drive gear configured to moveinto engagement with the linear drive mount upon extension of the shaft;wherein the first and second splines are configured to prevent jammingof the drive gear against the linear drive mount when the drive gearmoves into engagement with the linear drive mount upon extension of theshaft by providing rotational play between the first and second splines,the rotational play allowing a degree of free rotation of the drive gearrelative to the linear drive mount.
 13. The mobile robot of claim 12,wherein the mobile robot is configured to move up or down within thepassage by the rotation of the drive gear against the linear drivemount.
 14. The mobile robot of claim 12, wherein the drive gearcomprises a pinion gear and the linear drive mount comprises a rackcomprising teeth configured to mesh with the gears of the pinion. 15.The mobile robot of claim 14, wherein the pinion gear comprises gearteeth having chamfered lead in portions configured to facilitate meshingengagement of the pinion gear with the rack without jamming upon axialextension of the shaft.
 16. The mobile robot of claim 15, wherein therack comprises gear teeth having chamfered lead in portions configuredto facilitate meshing engagement of the pinion gear with the rackwithout jamming upon axial extension of the shaft.
 17. A method oftransporting a mobile robot along a vertical or inclined passage betweenstorage levels in an order fulfillment facility, the passage comprisinga linear drive mount, comprising the steps: a. extending a drive gear ofthe mobile robot from a first position spaced from the linear drivemount to a second position where the drive gear is engaged with thedrive mount, the drive gear mounted to a shaft; b. rotating the shaftand drive gear by a motor, the motor having a first spline and the shafthaving a second spline, the shaft rotating by a force exerted by thefirst spline on the second spline; and c. preventing improper seating ofthe drive gear against the linear drive mount by, at least in part,providing a degree of play between the first and second splines, theplay allowing a degree of free rotation of the drive gear relative tothe linear drive mount as the drive gear extends from the first positionto the second position.
 18. The method of claim 17, said step (c) ofpreventing improper seating of the drive gear against the linear drivemount further comprising the step of providing gear teeth on the drivegear with a chamfered lead in portion configured to facilitate meshingengagement of the drive gear with the linear drive mount upon movementof the drive gear from the first position to the second position. 19.The method of claim 17, further comprising the step of driving themobile robot to move up or down the passage by rotating the shaft anddrive gear while the drive gear is engaged with the linear drive mount.20. A method of transporting a mobile robot along a vertical or inclinedpassage between storage levels in an order fulfillment facility, thepassage comprising a linear drive mount, comprising the steps: d.extending a drive gear of the mobile robot toward the linear drive mountuntil the drive gear initially engages the linear drive mount, the drivegear mounted to a shaft; e. retracting the shaft and drive gear awayfrom the linear drive mount in the event of overlap or improper meshingof the drive gear with the linear drive mount; f. rotating the shaft anddrive gear an amount less than the pitch between gear teeth on the drivegear in the event of overlap or improper meshing of the drive gear withthe linear drive mount in said step (a); g. re-extending the drive gearand shaft toward the linear drive mount after said step (c) until thedrive gear again engages the linear drive mount in the event of overlapor improper meshing of the drive gear with the linear drive mount insaid step (a); and h. extending the drive gear into full meshingengagement with the linear drive mount in the event of proper meshing ofthe drive gear with the linear drive mount.
 21. The method of claim 20,further comprising the step of driving the mobile robot to move up ordown the passage by rotating the shaft and drive gear while the drivegear is in full meshing engagement with the linear drive mount.
 22. Themethod of claim 20, further comprising the step of chamfering edges ofthe teeth of the drive gear to reduce the likelihood of overlap orimproper meshing when the drive gear initially engages the linear drivemount.